Polyethylene terepthalate and its copolyesters (hereinafter referred to collectively as “PET”) are widely used for making containers for various packaged beverages, including carbonated beverages, juice, and water. Although PET has an excellent combination of clarity, mechanical, and gas barrier properties, use of PET for small packages of carbonated beverages and oxygen sensitive products is limited by PET's insufficient gas barrier properties to oxygen and carbon dioxide. Thus, there is a widespread need for a PET composition having improved gas barrier properties.
The packaging of carbonated beverages in small packages is limited by the high permeation rate of carbon dioxide through PET. Typically, packages have a permeation rate in the range of 3 to 14 cc/day, or a 1.5 to 2 percent loss per week, depending on the size of the package. The small package size has a larger surface-to-volume ratio than standard package sizes, resulting in a higher relative rate of loss. Therefore, PET containers primarily are used for large packaging of carbonated beverages, while metal cans and glass bottles are used for small packaging of carbonated beverages.
The amount of carbon dioxide remaining in a packaged carbonated soft drink determines its shelf life. Normally, carbonated soft drink containers are filled with approximately four volumes of carbon dioxide per volume of water. It is generally accepted that a packaged carbonated soft drink reaches the end of its shelf life when 17.5 percent of the carbon dioxide in the container is lost due to permeation of the carbon dioxide through the container side wall and closure. The permeability of PET to carbon dioxide therefore determines the shelf life of the packaged carbonated beverage and thus, the suitability of PET as a packaging material.
Numerous technologies have been developed or are being developed to enhance the barrier of PET to small gas molecules. For example, external or internal coatings for enhancing the gas barrier of PET containers have been developed. The coating layer is normally a very high barrier layer, either inorganic or organic, and slows down the diffusion of gases. Implementation of this technology, however, requires coating equipment not normally utilized in the manufacture of packaged beverages and therefore requires substantial capital investment, increased energy usage, and increased floor space. In many beverage packaging plants that are already crowded, the additional space is not an option.
Multi-layered containers also have been developed with a high barrier layer sandwiched between two or more PET layers. Implementation of this technology also requires substantial capital investment and delamination of the container layers impacts appearance, barrier, and mechanical performance of the containers.
A barrier additive for the PET or a polymer with inherent barrier properties would be preferred solutions. Neither such solution requires additional capital investment, and therefore, does not have the limitations inherent with other technologies. A barrier additive also can be added during the injection molding process which gives more flexibility for downstream operations.
L. M. Robeson and J. A. Faucher disclose in J. Polymer Science, Part B 7, 35-40 (1969) that certain additives can be incorporated into polymers to increase their modulus and gas barrier properties through an antiplasticization mechanism. This article discloses utilizing additives with polycarbonate, polyvinyl chloride, polyphenylene oxide, and polyethylene oxide.
In WO 01/12521, Plotzker et al. propose the use of additives selected from 4-hydroxybenzoates and related molecules to increase the gas barrier properties of PET. This published patent application discloses barrier additives of the following structures:
HO-AR-COOR, HO-AR-COOR1COO-AR—OH, HO-AR-CONHR,
HO-AR—CO—NHR3-COO-AR—OH, HO-AR—CONHR2NHCO-AR—OH
In the foregoing structure, AR is selected from substituted or unsubstituted phenylene or naphthalene and R1, R2, and R3 are selected from the group consisting of a C1 to C6 alkyl group, a phenyl group, and a naphthyl group.
The foregoing additives described in the art provide only moderate improvement in PET barrier, less than 2.1 times (X) for oxygen barrier for the best examples with a 5 weight percent loading level. At this loading level, however, PET experiences substantial degradation and a significant drop in intrinsic viscosity (IV). Although lowering the level of additive reduces the degradation of PET, it also reduces the barrier improvement factor, so much so that no real benefit exists in using these additives in packaging carbonated soft drinks or oxygen sensitive food. Part of the IV loss is due to the addition of the small molecular additive. Additional IV loss results when additives contain functional groups capable of reacting with PET and causing the break down of the molecular weight. Additives with reactive functional groups usually are more soluble in PET and thus do not impart haziness in the bottle. PET with a significantly lower IV cannot be used in blow molding containers, such as beverage containers. Furthermore, lower IV PET makes containers with poor mechanical performance, such as creep, drop impact, and the like. Still further, PET containers made from lower IV PET have poor stress cracking resistance, which is undesirable in container applications.
PET also has been modified or blended with other components to enhance the gas barrier of the PET. Examples include polyethylene naphthalate (PEN)/PET copolymers or blends, isophthalate (IPA) modified PET, PET blended with polyethylene isophthalate (PEI) or a polyamide, such as nylon, and PET modified with resorcinol based diols. For a PET copolymer to achieve moderate barrier enhancement of 2× or higher, the modification is normally more than 10 to 20 weight or mole percent of the total co-monomers. When PET is modified to such a high level, the stretching characteristics of the PET are changed dramatically such that the normal PET container preform design could not be used in the manufacture of containers. Using these PET copolymers to mold conventional PET container preforms results in preforms that can not be fully stretched and the ultimate containers are very difficult, if not impossible, to make. Even if such a container can be made, it does not show improved barrier performance and shows deteriorated physical performance such that it can not be used to package carbonated soft drinks. Furthermore, PET blends with polyamides, such as nylon, developed yellowness and haze and are not clear like conventional PET.
U.S. Pat. Nos. 5,888,598 and 6,150,450 disclose redesigned PET container preforms with thicker side walls to compensate for the increased stretch ratio. This thicker preform, however, requires new molds which require additional capital investment. The thicker preform also is made at a lower rate of productivity because it takes longer to cool and reheat the thicker wall perform during the blow molding process.
More recently, it has been discovered that low molecular weight compounds may provide the needed improvements to the gas barrier properties of PET. There is a significant problem with plate-out, however, for conventional injection molding of polymers having high melting and processing temperatures, such as PET, with low molecular weight additives. Plate-out occurs where there is deposition of material (e.g., polymer extracts, lubricants, stabilizers, or plasticizers) onto the surfaces of an injection molding apparatus during the processing of polymers. Plate-out reduces the running time of the injection molding apparatus, resulting in costly production delays for cleaning. Accordingly, there exists a need for an improved process and apparatus for injection molding polymers that reduces plate-out.